This patch adds two new APIs get_kernel_pages() and get_kernel_page()that may be used to pin a vector of kernel addresses for IO. The initialuser is expected to be NFS for allowing pages to be written to swapusing aops->direct_IO(). Strictly speaking, swap-over-NFS only needsto pin one page for IO but it makes sense to express the API in termsof a vector and add a helper for pinning single pages.